1. Field of the Invention
The present invention relates to a variable magnification optical system and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system usable in a video camera, an electronic still camera and the like, and appropriate especially for a surveillance camera, and also to an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, variable magnification optical systems for CCTV (Closed-circuit Television) were developed, as optical systems for imaging apparatuses, such as a video camera, an electronic still camera, and a surveillance camera, which use imaging devices, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), as recording media. As such variable magnification optical systems for CCTV, many optical systems with four group structure have been proposed, because they have many advantages, such as the simplicity of the lens barrel and variable magnification mechanisms and the easiness of handling (please refer, for example, to Japanese Unexamined Patent Publication No. 9(1997)-258102 (Patent Document 1), U.S. Pat. No. 6,512,637 (Patent Document 2), U.S. Patent Application Publication No. 20010019455 (Patent Document 3), Japanese Unexamined Patent Publication No. 2003-287678 (Patent Document 4), U.S. Patent Application Publication No. 20040042075 (Patent Document 5), Japanese Unexamined Patent Publication No. 2004-126631 (Patent Document 6), and Japanese Unexamined Patent Publication No. 2005-084409 (Patent Document 7)).
Further, although the structure of five-group optical systems is more complicated than the structure of four-group optical systems, many optical systems with five group structure have been also proposed to give excellent optical performance (please refer, for example, to U.S. Pat. No. 5,636,060 (Patent Document 8) and Japanese Unexamined Patent Publication No. 2011-081063 (Patent Document 9)).